Considering the different electric resistivity of subterranean hydrocarbons and formation water typically containing salts, resistivity surveys using electromagnetic (EM) fields penetrating into the formation are established as a valuable tool in the hydrocarbon industry. Such EM surveys can be performed to determine for example the presence of hydrocarbon reservoirs in geologic formations or to monitor the shifting positions of hydrocarbons and water during the course of hydrocarbon production and water flooding operations.
To determine the electrical resistivity of geologic formations surrounding and between boreholes drilled into the geologic formations of interest, deep-reading electromagnetic field surveys of subsurface areas typically involve large scale measurements from the surface, including the sea bottom, from surface-to-borehole, and/or between boreholes.
In two articles, “Crosshole electromagnetic tomography: A new technology for oil field characterization”, The Leading Edge, March 1995, by Wilt et al. and “Crosshole electromagnetic tomography: System design considerations and field results”, Society of Exploration Geophysics, Vol. 60, No. 3, 1995 by Wilt et al., the authors describe the principles guiding the measurement of geologic formation resistivity with low frequency electromagnetic (EM) systems.
Methods and tools for performing EM measurements are further described in a number of patents and patent applications including the co-owned U.S. Pat. No. 6,393,363 to Wilt and Nichols.
In “Using Crosswell Electromagnetic to Map Water Saturation and Formation Structure at Lost Hills”. SPE Western Regional Meeting, 26-30 Mar. 2001, Bakersfield, Calif. (SPE paper 68802) by M. Wilt et al., the authors describe a qualitative method of estimating the change in water saturation from time-lapse cross-well EM data.
Published International Patent Application WO2001/020366 provides further background related to reservoir resistivity mapping with deep electromagnetic measurement. It also teaches the combination or joint inversion of resistivity depth images with other geological and geophysical data to estimate the reservoir properties.
The co-owned U.S. Pat. No. 7,363,164 to Little and LaVigne describes Archie and Waxman-Smits laws in the context of using dual and triple water models to interpret electrical resistivity measurements. The modeling of the intrusion of salty water into a medium (Henry problem) is described for example by A. E. Croucher and M. J. O'Sullivan in: ‘The Henry problem for saltwater intrusion’, Water Resources Research, 31(7), 1995, pp 1809-1814. A solution of this Henry problem is implemented as the so-called ‘Groundwater model’ within Eclipse™, which is a well-established commercially available reservoir simulation software.
In view of the known art, it is seen as an object of the invention to improve and enhance knowledge of reservoir parameters as gained from deep electromagnetic surveys.